The invention relates to a method for estimating channel bit error ratio in a receiver, the method comprising: detecting a data sequence of a received signal; decoding a first encoding of the detected data sequence; re-encoding the data sequence decoded from the first encoding by applying the first encoding.
Communication systems, such as cellular radio networks, need information about the quality of the transmission channel for system tuning. A receiver typically informs the quality of the channel it receives to a transmitter. In the GSM system, for example, one method of measuring channel quality is to determine its bit error ratio. The real bit error ratio (bit error rate) of a channel cannot be measured without knowing exactly what was sent.
A prior art method to estimate channel bit error ratio is to use a re-encoding technique, whereby a decoded data sequence is re-encoded. The re-encoded data sequence is compared with the detected data sequence to allow the bit error ratio of the channel concerned to be estimated. The comparison is carried out by measuring how much the re-encoded sequence differs from the received detected sequence. The estimate of the channel bit error ratio thus obtained is referred to as a pseudo bit error ratio.
If the decoder has been able to correct all the errors that have appeared into the bit in the channel, the bit error ratio estimate provides an accurate value for the real bit error ratio. Any errors left after the decoding diminish the accuracy the bit error ratio estimate. The more the sequence decoded from the channel encoding contains errors, the less useful the calculated estimate will be. This leads to situations where the pseudo bit error ratio can erroneously indicate a bad channel to be a good one. The estimating of the bit error ratio is based on the assumption that the decoded data does not contain errors, or at least there are very few of them.
A widely used coding method is convolution coding, in which the data to be transmitted is coded into code words. In the channel there are sources of interference distorting the code words. In the receiver the received information no longer corresponds to any accepted code word. It is therefore possible that an incorrectly received, distorted code word can be interpreted as another code word than the one that was sent. As the incorrect code word is then re-encoded and compared with the distorted code word received, the pseudo bit error ratio provided is a number representing a channel of good quality. The estimation is, however, incorrect because the right object of comparison would have been the code word that was actually sent. If the code word that was actually sent were known, it could be compared with the distorted code word received, and the pseudo bit error ratio obtained as a result would be a number representing a channel of a poor quality, which would be correspond to reality.
FI 934480 and U.S. Pat. No. 5,113,400 disclose decoders using the re-encoding method. In FI 934480 the estimating of the pseudo bit error ratio and the estimated signal-to-noise ratio are combined into one equation. The value provided by the equation is compared with a predetermined threshold value, whereby it is possible to conclude whether the quality of the GSM speech frame concerned is poor. The disclosed method cannot not, however, be used for enhancing the accuracy of the estimating of the pseudo bit error ratio. U.S. Pat. No. 5,113,400 discloses a error detection method applying the above described re-encoding method. The re-encoding method described in the publication incorporates the shortcomings already described, and it does not offer solution to the problem concerned, i.e. how to obtain a reliable pseudo bit error ratio also in situations where the decoded data sequence received contains errors.
An object of the invention is thus to provide a method and an equipment implementing the method which allow the above problems to the solved. This is achieved with a method described in the preamble, characterized by determining a value for the quality of the detected data sequence and, if the quality of the detected data sequence fulfils a predetermined quality requirement, by estimating a bit error ratio by comparing the detected data sequence with the data sequence re-encoded by applying the first encoding.
The invention further relates to a receiver comprising: detecting means for detecting a data sequence of a received signal; decoding means for decoding a first encoding of the detected data sequence; re-encoding means for re-encoding with the first encoding the data sequence decoded from the first encoding.
The receiver of the invention is characterized in that the receiver further comprises: quality determining means for providing a value for the quality of the detected data sequence; estimating means for estimating a bit error ratio, if the quality of the detected data sequence fulfils a predetermined quality requirement, by comparing the detected data sequence with the data sequence re-encoded with the first encoding.
Preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea of not estimating bit error ratio for a data sequence if by means of another method it has been shown that the data sequence does not fulfil a predetermined quality requirement.
An advantage the method and system of the invention provide is that on a longer term they improve the accuracy of pseudo bit error ratio in situations where the decoding of the first encoding cannot remove all errors.